Study yields better turbine spacing for large wind farms

Large wind farms are being built around the world as a cleaner
way to generate electricity, but operators are still searching for
the most efficient way to arrange the massive turbines that turn
moving air into power.

To help steer wind farm owners in the right direction, Charles
Meneveau, a Johns Hopkins fluid mechanics and turbulence expert,
working with a colleague in Belgium, has devised a new formula
through which the optimal spacing for a large array of turbines can
be obtained.

"I believe our results are quite robust," said Meneveau, who is
the Louis Sardella Professor of Mechanical Engineering in the
university's Whiting School of Engineering. "They indicate that
large wind farm operators are going to have to space their turbines
farther apart."

The newest wind farms, which can be located on land or offshore,
typically use turbines with rotor diameters of about 300 feet.
Currently, turbines on these large wind farms are spaced about
seven rotor diameters apart. The new spacing model developed by
Meneveau and Johan Meyers, an assistant professor at Katholieke
Universiteit Leuven in Belgium, suggests that placing the wind
turbines 15 rotor diameters apart -- more than twice as far apart
as in the current layouts -- results in more cost-efficient power
generation.

Meneveau presented the study results recently at a meeting of
the American Physical Society Division of Fluid Dynamics. Meyers,
co-author of the study, was unable to attend.

The research is important because large wind farms —
consisting of hundreds or even thousands of turbines — are
planned or already operating in the western United States, Europe
and China. "The early experience is that they are producing less
power than expected," Meneveau said. "Some of these projects are
underperforming."

Earlier computational models for large wind farm layouts were
based on simply adding up what happens in the wakes of single wind
turbines, Meneveau said. The new spacing model, he said, takes into
account interaction of arrays of turbines with the entire
atmospheric wind flow.

Meneveau and Meyers argue that the energy generated in a large
wind farm has less to do with horizontal winds and is more
dependent on the strong winds that the turbulence created by the
tall turbines pulls down from higher up in the atmosphere. Using
insights gleaned from high-performance computer simulations as well
as from wind tunnel experiments, they determined that in the
correct spacing, the turbines alter the landscape in a way that
creates turbulence, which stirs the air and helps draw more
powerful kinetic energy from higher altitudes.

The experiments were conducted in the Johns Hopkins wind tunnel,
which uses a large fan to generate a stream of air. Before it
enters the testing area, the air passes through an "active grid," a
curtain of perforated plates that rotate randomly and create
turbulence so that the air moving through the tunnel more closely
resembles real-life wind conditions.

Air currents in the tunnel pass through a series of small
three-bladed model wind turbines mounted atop posts, mimicking an
array of full-size wind turbines. Data concerning the interaction
of the air currents and the model turbines is collected by using a
measurement procedure called stereo particle-image-velocimetry,
which requires a pair of high-resolution digital cameras, smoke and
laser pulses.

Further research is needed, Meneveau said, to learn how varying
temperatures can affect the generation of power on large wind
farms. The Johns Hopkins professor has applied for continued
funding to conduct such studies.